In recent years, with the increase in data processing speed of calculating equipment such as a computer, decrease in the dielectric constant of a printed circuit board is demanded.
A printed substrate used for a usual printed circuit board is fabricated by impregnating a base material such as a fiberglass cloth or a paper with resin material. Accordingly, in decreasing the dielectric constant of a printed circuit board, effective is decrease in the dielectric constant of a fiberglass cloth or of a resin.
Further, in recent years, with the increase in scale of an LSI (Large Scale Integration), degradation in reliability of a connection between a BGA (Ball Grid Array) package and a printed circuit board owing to the difference between their thermal expansion coefficients has become a problem. For the purpose of improving such connection reliability, demanded is decrease in the thermal expansion coefficient of a printed circuit board.
As a method of fabricating a printed circuit board by the use of a low dielectric constant resin, it is considered, for example, to use the low dielectric constant resin only for a layer for transmitting a high speed signal and use a usual resin for the other layers. The cost of such a printed circuit board using a low dielectric constant resin only for a layer for transmitting a high speed signal and using a resin with a usual dielectric constant for the other layers is lower than that of a printed circuit board with all layers made of the low dielectric constant resin.
However, in the case of such a printed circuit board using a low dielectric constant resin only for a layer for transmitting a high speed signal and a usual resin for the other layers, the use of different resins in the fabrication process lowers the productivity.
As a fiberglass cloth with a lower dielectric constant and a lower thermal expansion coefficient compared to that made of E-glass, which is a usual 10 fiberglass cloth, for example, there are fiberglass cloths made of D-glass, S-glass and the like. However, the use of these fiberglass cloths has a problem in that the cost of substrate material becomes high. Additionally, D-glass has a problem with drill processability such as wear of a drill and hole positioning accuracy in drilling.
Patent Document 1 (Japanese Laid-open Patent Application No. 2000-234239) discloses, as a substrate for a printed circuit board with low dielectric constant, low cost and excellent drill processability, a substrate using a fiberglass cloth in which either of its warp and weft is made from E-glass and the other from D-glass. Through such combining of different types of glass yarn, the fiberglass cloth of Patent Document 1 can be given a different property from the original one. Further, by fabricating a printed circuit board with the fiberglass cloth of Patent Document 1, decrease in dielectric constant and that in thermal expansion coefficient become possible.
However, even in the case of using a fiberglass cloth consisting of different types of glass yarn such as in Patent Document 1, there is no change in the fact that a high cost material with a low dielectric constant or a low thermal expansion coefficient is used as the warp or the weft. Accordingly, the fiberglass cloth of Patent Document 1 has a problem in that it costs higher than a usual fiberglass cloth.
Further, the printed circuit board according to Patent Document 1 has a structure consisting of laminated fiberglass cloths each containing low dielectric constant glass yarn with poor drill processability. Accordingly, the fiberglass cloth of Patent Document 1 has also a problem with wear of a drill and hole positioning accuracy in drilling, if compared with a usual fiberglass cloth.
The objective of the present invention is to provide a printed circuit board which enables transmission of a high speed signal, with low cost and with no degradation in processability.